The present invention relates to the field of materials analysis and, more particularly, to analyzing the grain structure of crystalline materials.
The complexity of the preferred orientation of polycrystalline microstructures has increasing significance within many industrialized fields. This microstructure can be examined with a variety of techniques. Multiphase two-dimensional mapping of crystallographic and morphological data provides challenges to determine the crystallographic grain orientation, grain size and grain boundaries of a crystalline sample. There are numerous ways of obtaining this information, but each of the methods presents slightly different information that the others do not.
The processing of materials in the semiconductor industry to achieve smaller geometries introduces new problems as the boundaries between grain structures and the orientations of the boundaries become more critical. For example, the conventional method of indexing Kikuchi diffraction patterns over a scanned area is one method of determining both the crystallographic orientation and grain morphology of thin films on sample surfaces. Backscattering Kikuchi Diffraction (BKD) in a scanning electron microscope can produce Kikuchi bands from polycrystalline grains approaching the size of the probe diameter. By applying the rules of point group symmetry to the Kikuchi bands, characteristics such as crystallographic grain orientation and grain size within a specimen can be determined.
Grains within polycrystalline materials generally have orientations that vary from grain to grain. This variation, when considered over a bulk specimen area, can lead to the directional grouping of specific crystalline planes with respect to certain crystallographic axes. The xe2x80x9cpreferred orientationxe2x80x9d of a polycrystalline sample refers to an average, or overall, orientation of the grains. The complexity of the preferred orientation of polycrystalline microstructures can be examined with a technique known as Orientation Imaging Microscopy, which analyzes collections of BKD patterns. This technique combines the advantages of point orientation in Transmission Electron Microscopy (TEM) with morphological information over a large enough area to provide statistical relevance.
Aluminum deposited by chemical vapor deposition (CVD) deposits in a preferred orientation along a (1,1,1) fiber texture normal to a silicon substrate. This geometry is preferred to reduce electromigration. BKD pattern analysis can be used to quantify the quality of the deposition of the aluminum along the preferential crystallographic axis.
The movement of the semiconductor industry to copper metallization will require seed layers and barrier layers made out of tantalum nitride, for example. The deposition of copper by CVD does not seem to exhibit preferential orientation. This results in a variable that can differ between deposited copper films. BKD analysis provides a way of quantifying the films for orientation analysis in a two-dimensional mapping array whereby the preferred grain orientations can be compared from one film to another.
BKD pattern analysis works by collecting a Kikuchi pattern at a specific location on a sample surface, converting the pattern to a Hough space where each line is represented as a spot, and using the angular deviations between the spots to calculate the crystallographic orientation of the crystal at that location. The scanning electron microscope beam or the sample stage is then stepped to the next point and the process is repeated. The stepping occurs in a raster pattern with a fixed step size over the entire scan area. Unfortunately, this method is very time consuming. For example, to acquire a pattern from an area that is 10 square micrometers with a step size of 50 nm, approximately 40,000 individual Kikuchi patterns must be collected and analyzed. With each Kikuchi pattern typically taking approximately 0.5 seconds, this yields a scan time for the entire area of approximately 11 hours.
The pattern also has a maximum grain boundary resolution of 50 nm. The lengthy collection time of these patterns makes automated BKD pattern analysis labor intensive and time consuming. Increasing the step size does decrease the time element involved in obtaining and analyzing date with respect to certain characteristics of a polycrystalline material.
The foregoing metrological techniques are conducted off-line, i.e., by taking partially fabricated structures in fabrication, including semiconductor devices, out of the manufacturing sequence. However, inline metrology techniques that identify either grain size or preferred orientation of polycrystalline films do not exist. Semiconductor devices are typically destructively measured offline by time consuming techniques of electron diffraction and x-ray diffraction. The disadvantage of these offline techniques is that they require constant monitoring on test structures and wafers, which results in a window between when problems occur and when problems are detected.
The present invention provides an apparatus for scanning a crystalline sample, including a sample holder, an electron source for generating an electron beam, and a scanning actuator for controlling relative movement between the electron beam and the crystalline sample. The scanning actuator is preferably controllable for directing the electron beam at a series of spaced apart points of the crystalline sample. Moreover, the apparatus also preferably includes an image processor for processing an image based upon electrons from the crystalline sample, and a controller for controlling the scanning actuator to maintain a distance between the points such that each image processed is representative of a different grain of the crystalline sample. Accordingly, the number of points required by the present invention may be significantly less than the number of points required for conventional fixed spaced systems, thereby significantly reducing the time for scanning the crystalline sample.
The image is preferably a Kikuchi diffraction pattern. The sample holder preferably holds the crystalline sample in a substantially horizontal position. The electron source is positioned such that an electron beam generated therefrom is at an angle approximately 20xc2x0 above horizontal. Furthermore, the apparatus preferably includes a phospor screen adjacent the sample holder, at a right angle incident to the electron beam, for forming the image defined by the electrons from the crystalline sample. The image processor may include a low light camera or a CCD camera for capturing the image defined by electrons from the crystalline sample. Also, the image processor may convert the Kikuchi diffraction pattern to a Hough space to identify Kikuchi bands at a point within the crystalline sample. The image processor may determine a crystallographic grain orientation at the point within the crystalline sample based on the Kikuchi bands. The controller determines an average crystallographic grain orientation for the crystalline sample from the processed images, and monitors any variance in the average grain orientation during the scanning of the crystalline. When the variance in the average grain orientation approaches a predetermined value, the scanning of the crystalline sample is terminated.
The objects, features and advantages in accordance with the present invention are provided by a method including the steps of providing the crystalline sample, generating an electron beam, and controlling relative movement between the electron beam and the crystalline sample to direct the electron beam at a series of spaced apart points of the crystalline sample. Furthermore, an image based upon electrons from the crystalline sample is processed, and a spacing between points is maintained so that each point is representative of a different grain of the crystalline sample.
In a preferred embodiment, the present invention is integrated in the fabrication process of semiconductors as an in-line method of scanning the crystalline materials. The present invention provides a means of testing the quality of device films real time, to identify problems during the manufacture of integrated circuits. An ability to monitor the metrology and/or morphology of the crystalline specimens xe2x80x9cin-linexe2x80x9d enables one to identify issues xe2x80x9cin-linexe2x80x9d, and extend the quality of the product, reduce scrap while increasing yield of a product.